Hmm that's strange. Do you mind observing the output of the comparator while you're increasing the bias from 0mv to 500mv? Since we know it "works" at 0V bias there should be a certain bias voltage where it stops working. This might tell us something.

If it does not work at 0mv bias (original setup) then the ESD diodes are likely blown and the analog input will be undefined. If the ESD diode is blown in such a way that it acts as a short circuit to GND then Vin - will be at 0V, and Vin + > Vin - will always be true.